1. Field of the Invention
The present invention relates to a video signal processor and, more particularly, to a video signal processor for reproducing a MUSE (Multiple Sub-Nyquist Sampling Encoding) signal received by a television set of a MUSE system as the original Hi-vision (high-definition) signal.
2. Description of the Related Art
Development of Hi-vision (high-definition television) broadcasting is also in progress in Japan and has already reached the stage of practical use. In order to transmit Hi-vision signals, what is called a MUSE (Multiple Sub-Nyquist Sampling Encoding) system is adopted.
A frequency band for Hi-vision signals is about five times of the frequency band for signals of an NTSC system. In order to transmit Hi-vision signals as they are, a signal frequency band of about 20 to 25 MHz is necessary. However, since the bandwidth of radio waves which is practically usable has a limitation, it is impossible to transmit the signals in such a wide frequency band on a radio wave as they are for broadcasting. It is therefore necessary to compress the bandwidth of a Hi-vision signal.
The MUSE system is a broadcasting system which has been developed in order to broadcast by transmitting Hi-vision signals from a satellite through one channel. In the MUSE system, basebandwidth compression is realized by a comparatively simple system without impairing the picture quality of television. In the MUSE system, the frequency of a baseband signal containing a brightness signal having a bandwidth of 22 MHz and a color signal having a bandwidth of 7 MHz is modulated and this baseband signal is transmitted through one channel having a frequency of 27 MHz, which is a bandwidth allotted to the satellite broadcasting in Japan. In the case of adopting a frequency modulation system as a modulation system for signals, since it is necessary to allow for adequate frequency deviation, the bandwidth of a transmission signal must be compressed to about 1/3 of the bandwidth of a radio wave. In the MUSE system, the bandwidth of the baseband signal is compressed to about 8.1 MHz.
The bandwidth compression in the MUSE system is carried out by thinning out the complete group of sampling points which are extracted from the original Hi-vision signal by an encoder on the transmission side in accordance with a predetermined rule. The thinning-out of the sampling points is carried out by utilizing a physiological characteristic of a television viewer. That is, by utilizing the physiological characteristic of the viewer that the resolution in an oblique direction is lowered than that in a horizontal or vertical direction, interfield offset sampling is carried out. The physiological characteristic of the viewer is also utilized that in the area in which the image is moving, the deterioration of the picture quality is not so conspicuous to the viewer even if the resolution is slightly lowered.
A decoder on the reception side fills up the sampling points taken out by the encoder on the transmission side by what is called interpolation. That is, the sampling points actually supplied from the encoder are received and the sampling points taken out are reproduced on the basis of the groups of the sampling points therebefore and thereafter and inserted into the vacant portions.
FIG. 11 is a block diagram of a general structure of a conventional video signal processor. In FIG. 11, a conventional video signal processor is composed of input terminals 11 to 13, an inverse matrix circuit 14, a high-pass filter 15, a color signal interpolator 16, adders 17 to 19, a gamma correcting portion 21, a time base expander 23, a digital/analog (D/A) converting portion 25, low-pass filters (LPF) 26 to 28, and output terminals 31 to 33.
Among the input terminals 11 to 13, the input terminal 11 is a Y signal input terminal to which a digital brightness signal (Y signal) is input, the input terminal 12 is an R-Y signal input terminal to which a digital color difference signal (R-Y signal) is input and the input terminal 13 is a B-Y signal input terminal to which a digital color difference signal (B-Y signal) is input. The Y signal input terminal 11 is directly connected to the inverse matrix circuit 14 and the high-pass filter 15. The R-Y signal input terminal 12 and the B-Y signal input terminal 13 are connected to the inverse matrix circuit through the color signal interpolator 16.
R, G and B signals output from the inverse matrix circuit 14 are input to the adders 17 to 19, respectively. The adders 17 to 19 add the respective outputs of the inverse matrix circuit 14 to the output of the high-pass filter 15. The outputs of the adders 17 to 19 are connected to the time base expander 23 through the gamma correctors 21-1 to 21-3, respectively, of the gamma correcting portion 21. The R, G and B signals which are subjected to time base expansion by use of the time base expander 23 are converted to analog R, B and G signals, respectively, by D/A converters 25-1 to 25-3 of the digital/analog converting portion 25. The R, B and G signals converted by the D/A converters 25-1 to 25-3 are output from the output terminals 31 to 33 through the low-pass filters 26 to 28, respectively.
The operation of a conventional video signal processor having the above-described structure will now be explained. The Y signal which are decoded from the MUSE signal and having a propagation rate of 48.6 Mbps is input to the Y signal input terminal 11. Similarly, the R-Y signal and B-Y signal having a propagation rate of 16.2 Mbps are input to the R-Y signal input terminal 12 and the B-Y signal input terminal 13, respectively. The Y signal is input to the high-pass filter 15 and the high-frequency component thereof is extracted in accordance with the transfer characteristic represented by the following equation (1): EQU H0(Z0.sup.-1)=k[1-(Z0.sup.-1 +Z0)/2] (1)
wherein Z0 is a function of a horizontal frequency f represented by the following equation (2): EQU Z0=exp(j2.pi.f/f0) (2)
wherein k is a positive constant and f0 is 48.6 MHz.
The color signal interpolator 16 to which the R-Y signal and the B-Y signal are input from the input terminals 12 and 13, respectively, interpolates the respective signals and outputs the R-Y signal and the B-Y signal each having a propagation rate of 16.2 Mbps in the form of the R-Y signal and the B-Y signal each having a propagation rate of 48.6 Mbps. The inverse matrix circuit 14 calculates the Y signal input from the Y signal input terminal 11 and the R-Y signal and the B-Y signal each having a propagation rate of 48.6 Mbps and output from the color signal interpolator 16 in accordance with the following equation (3) and outputs the R, G and B signals each having a propagation rate of 48.6 Mbps. ##EQU1##
The high-frequency component of the Y signal having a propagation rate of 48.6 Mbps which is extracted by the high-pass filter 15 is added to the R, G and B signals having a propagation rate of 48.6 Mbps which are output from the inverse matrix circuit 14 by the adders 17 to 19. The R, G and B signals having a propagation rate of 48.6 Mbps with the contours corrected are thus output from the adders 17 to 19. The R, G and B signals output from the adders 17 to 19 are subjected to gamma correction by use of the gamma correctors 21-1 to 21-3. The gamma correction portion 21 outputs the R, G and B signals having a propagation rate of 48.6 Mbps with the contours and the gamma corrected. The R, G and B signals are input to the time base expander 23.
The time base expander 23 expands the time base of the signals to 12/11 which are transmitted from a transmitter with the time base compressed to 11/12. The R, G and B signals having a propagation rate of 48.6 Mbps are converted into the R, G and B signals having a propagation rate of 44.5 Mbps by the time base expander 23 and input to the D/A converting portion 25.
The digital R, G and B signals input to the D/A converting portion 25 are converted into analog signals by the D/A converters 25-1 to 25-3. The R, G and B signals converted into analog signals are input to the low-pass filters 26 to 28. The low-pass filters 26 to 28 pass only the horizontal frequencies in a low frequency band of the analog R, G and B signals while limiting the higher cutoff frequencies, thereby removing the aliasing noise component. The output characteristic of the thus-reproduced signal is expressed by the hatched portion in FIG. 4.
In this way, the analog R, G and B signals reproduced are output from the output terminals 31 to 33, respectively.
FIG. 12 is a block diagram of a general structure of a conventional color signal interpolator. The conventional color signal interpolator is composed of an interframe interpolated data input terminal 34, a field memory 35, a line memory 36, a first selector 37, a second selector 38 and an interframe interpolated data output terminal 39. Data on the color signal which is supplied to the color signal interpolator and interpolated between frames in the MUSE system is input to the interframe interpolated data input terminal 34. The color signal output from the interframe interpolated data output terminal 34 is input to the field memory 35, which delays the color signal by one field before outputting it to the line memory 36. The line memory 36 delays the output signal of the field memory by one line and outputs it to the first selector 37. The first selector 37 inputs the output signal of the field memory 35 and the output signal of the line memory 36 and outputs them to the second selector 38 alternately with a period of one field. The second selector 38 alternately outputs the output signal from the interframe interpolated data input terminal 34 and the output signal from the first selector 37 with a period of a frequency F which is equivalent to half of the period of the interframe interpolated data. The interframe interpolated data output terminal 39 leads the signal output from the second selector 38 to the outside of the color signal interpolator as a color signal interpolated between fields.
The operation of the conventional color signal interpolator having the above-described structure will now be explained.
FIG. 13 is an explanatory view of the sampling patterns for processing the color signals for a still picture in the MUSE system. As shown in FIG. 13, since two kinds of color signal are transmitted line sequentially in the MUSE system, each of the color signals are thinned out on every other line.
In FIG. 13, if the intervals T1, T2 and T3 between the represented by the marks .largecircle. and .DELTA., and and are represented by frequencies, T1 is equivalent to F1=16.2 (MHz), T2 is equivalent to F2=8.1 (MHz) and T3 is equivalent to F3=4.05 (MHz). F1 is equal to the frequency F which corresponds to half of the period of the interframe interpolated data.
Under the above-described conditions, the data rate of the original transmitted signal is F3 and the sampling points of the transmission signal are offset at an interval of T2 on every other line, namely, in every frame. The sampling points of the transmission signal are also offset at an interval of T1 in every field, so that the sampling points are restored to the original in four fields.
The signal supplied to the interframe interpolated data input terminals 34 is a signal reproduced by inserting the signal transmitted one frame before between the sampling points of a transmitted signal. The data rate of the signal is T2. The signal supplied to the interframe interpolated data input terminals 34 is composed of .largecircle. and .DELTA. or and shown in FIG. 13. The field memory 35 delays the signal supplied to the interframe interpolated data input terminal 34 by the time equivalent to 562 horizontal scanning (hereinunder referred to as "H") periods, namely, one field. The line memory 36 delays the output signal of the filed memory 35 by the time equivalent to 1 H period, namely one line.
It is obvious from FIG. 13 that if the signal supplied to the interframe interpolated data input terminal 34 is composed of .largecircle. and .DELTA., in other words, the current field is a first field or a third field, the signal on the closest line to the signal supplied to the interframe interpolated data input terminal 34 among the signals composed of and and transmitted one field before is the signal transmitted 563 H before.
Therefore, if the output signal of the line memory 36 is selected by the first selector 37 and the signal supplied to the interframe interpolated data input terminal 34 is changed over to the output signal of the first selector 37 at a rate of F1, namely, the frequency F which is equivalent to half the period of the interframe interpolated data, the signal composed of .largecircle. and .DELTA. with the closes and inserted therebetween is obtained at the output of the second selector 38 as the color signal interpolated between fields.
Similarly it is obvious from FIG. 13 that if the signal supplied to the interframe interpolated data input terminal 34 is composed of and , in other words, the current field is a second field or a fourth field, the signal on the closest line to the signal supplied to the interframe interpolated data input terminal 34 among the signals composed of .largecircle. and .DELTA. and transmitted one field before is the signal transmitted 562 H before.
Therefore, if the output signal of the field memory 35 is selected by the first selector 37 and the signal supplied to the interframe interpolated data input terminal 34 is changed over to the output signal of the first selector 37 at a rate of T1, namely, the frequency F which is equivalent to half the period of the interframe interpolated data by the second selector 38, the signal composed of and with the closest .largecircle. and .DELTA. inserted therebetween is obtained at the output of the second selector 38 as the color signal interpolated between fields.
In this way, interfield interpolated data is obtained at the interfield interpolated data output terminal 39 for leading the output of the second selector 38 at a data rate of a frequency of F, which is equivalent to half the period of the interframe interpolated data, namely, a period of T1. The interfield interpolated data output to the interfield interpolated data output terminal 39 is shown in FIG. 14.
In the conventional video signal processor having the above-described structure, it is necessary to process a signal at a digital portion at a high speed. In order to process a digital signal at a high speed, a large load is applied to the hardware. It is therefore necessary that the conventional video signal processor is composed of elements which operate at a high speed. It is also necessary to use a low-pass filter having a sharp cutoff characteristic in the process of cutting off the high-frequency component of the output signal after the D/A conversion. Since such elements which operate at a high speed and a low-pass filter having a sharp cutoff frequency are generally expensive, it is difficult to reduce the cost of the circuit as a whole in the conventional video signal processor.
The conventional color signal interpolator suffers from a problem of vertical line disturbance due to the variation in the clamping level. If there is a deviation of a direct current level of the signal of and which corresponds to the signal of .largecircle. and .DELTA. in the output signal interpolated between fields such as that shown in FIG. 14, a vertical line disturbance at a rate which corresponds to the period of T1 is caused. In order to prevent this, the variation in the clamping level between fields is hardly permitted. Under such restriction, good processing of color signals for a still picture is impossible.